Preprocessor output

Question

How do I view the output produced by the C pre-processor, prior to its conversion into an object file?

I want to see what the MACRO definitions do to my code.

score 103 · Accepted Answer · answered Sep 18 '10 at 18:00

103

gcc -E file.c

or

g++ -E file.cpp

will do this for you. The -E switch forces the compiler to stop after the preprocessing phase, spitting all it’s got at the moment to standard output.

Note: Surely you must have some #include directives. The included files get preprocessed, too, so you might get lots of output.

For Visual C++ the switch is /E which spits the preprocessor output to screen.

answered Sep 18 '10 at 18:00

Faisal Feroz

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`gcc -E -nostdinc` will bypass standard #include files – pmg Sep 18 '10 at 18:03
3

You might also need to specify `-x c` if the file you are processing has not a standard c extension – Antonio Apr 13 '17 at 09:26
2

You might need to specify `-I`, and `-D` switches. – x-yuri Feb 28 '18 at 19:11
5

The `-P` flag will Inhibit the generation of linemarkers. – user2023370 Sep 17 '20 at 22:40

score 17 · Answer 2 · answered May 04 '15 at 14:48

17

You can also call the C Preprocessor directly.

cpp infile outfile

Check out man cpp for more info.

answered May 04 '15 at 14:48

rdas

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Nice! I prefer this one over `cc -E`. Also, I use `-P` to disable line markets too (works with `cc` as well). – ziya Feb 21 '20 at 18:13

score 9 · Answer 3 · answered Oct 02 '13 at 11:26

For GCC,

gcc -E -dM file.c

or

g++ -E -dM file.cpp

should do the job. -dM, as GNU Preprocessor manual puts it, should generate a list of ‘#define’ directives for all the macros defined during the execution of the preprocessor, including predefined macros.

score 3 · Answer 4 · answered Sep 18 '10 at 18:00

3

It depends on the compiler you use.
With GCC, you can specify the -E flag on the command-line to let the compiler produce the pre-processor output.

answered Sep 18 '10 at 18:00

Bart van Ingen Schenau

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score 1 · Answer 5 · answered Jan 31 '21 at 13:53

If using CLion by Jetbrains, you can use the action "clangd: Preprocess current TU"

So hit shift shift and start typing clangd...

Best assign it to a shortcut for simpler reuse in preferences->keymap:

Shout out to marcosbento

PS: TU means 'translation unit' (see here LLVM translation unit)

score 0 · Answer 6 · answered Dec 03 '13 at 16:11

You can check out my script described here:

http://mosermichael.github.io/cstuff/all/projects/2011/09/16/preprocessor.html

It formats the preprocessor output into a (hopefully) readable html document: lines that are different due to preprocessor are marked in the file.

Preprocessor output

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